It's my understanding that most of these are driven by either overfilled oil or hydraulic fluids. AA had a pretty big push on this a few years ago that resulting in some actions being taken to curb the events.

Also, we are all breathing fumes at low concentrations as seals are never 100% perfect especially at higher pressures. Some oil and hydraulic fluid is always getting atomized into the pack air we breathe.

I read a study on ALS and pilots were like the #2 or #3 profession to get it, it's a terrible disease. Truck drivers as well. Something about the diesel particulate in the exhaust. An increase of ~50% from the baseline.